During the past year, our research has continued its investigations of central nervous system (CNS) serotonin correlates of PET activity. A new study was initiated that was designed to assess serotonin correlates of intrinsic tolerance in alcohol naive rhesus macaques. To assess the role of serotonin functioning on excessive alcohol consumption and the related high levels of aggression, alcohol naive rhesus macaque subjects were given identical doses of IV alcohol and rated for levels of intoxication. Result showed that there was a nearly significant positive correlation between levels of intoxication and CSF 5-HIAA concentrations. Following the ratings for intoxication, an experimenter blind to the subjects' intoxication level and CSF 5-HIAA concentrations entered the room and rates of aggression by the subjects to the experimenter were recorded. Subjects with lower intoxication levels and low CSF 5-HIAA were more likely to aggress at the experimenter.